Musnad Ahmad 1007:82

Bishr ibn Shu‘ayb narrated to us, my father narrated to me, from al-Zuhri, Abu Salamah ibn ‘Abd al-Rahman informed me that Abu Qatadah was one of the companions of the Messenger of Allah, may Allah bless him and grant him peace, and one of his horsemen. He said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "The true dream is from Allah, and the nightmare is from Satan. So if one of you has a dream he dislikes, let him spit to his left three times, and let him seek refuge in Allah from it, for it will not harm him."